The proposed experiments are designed to investigate the limited protective value of the primary and secondary immune responses to airborne infection with virulent M. tuberculosis (Mtb) in mice. The secondary response will be studied in mice that possess a state of immunological memory as a result of having been infected with avirulent or virulent live Mtb and later treated with chemotherapy to essentially abolish infection. Vaccinated and unvaccinated mice will be infected with virulent Mtb by aerosol and 5 mice sacrificed at progressive times of infection to follow the progression and subsequent immunological control of infection. Additional mice will be sacrificed to determine the kinetics of production in the draining tracheobronchial lymph node of Mtb-specific CD4 Th1 cells and CD8 T cells, identified and enumerated by their ability to secrete IFNgamma in response to Ag stimulation in the ELISPOT assay. The appearance of these T cells in the draining nodes will be compared with their appearance in blood and at sites in infection in the lungs. It will be determined, with the RNase protection assay and Western blotting, whether the onset of expression of immunity in the lung is associated with transcriptional activation of genes for Th1 cytokines needed for the mediation of immunity, and if iNOS needed for the expression of immunity by macrophages at infectious foci. The prediction that neither the primary, nor the secondary immune response will be capable of preventing the establishment of infection even with very small numbers of virulent Mtb, or even with a virulent Mtb, because of an intrinsic delay before immunity can be expressed in the lungs will be tested. The prediction that, once expressed, immunity will be unable to resolve even very low levels of infection, but will cause infection to become stationary, will also be tested by challenging mice with small numbers of Mtb, and by later reducing the number of Mtb in lesions with chemotherapy as immunity is being expressed. Whether the delay in expression of immunity is caused by a delay in the production of protective T cells, or a delay in the development of conditions at infectious foci necessary for the extravasation of T cells at these sites will be investigated by adoptive transfer studies. The inability of the secondary immune response to enable genetically susceptible mice to stop lung infection from progressing will be investigated. The possibility that genetically susceptible mice, will be incapable of stopping lethal regrowth of a greatly reduced level of lung infection following chemotherapy will also be investigated.